Cap Forming Procedure for 12xxx Build

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YMI5150?
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Cap Forming Procedure for 12xxx Build

Post by YMI5150? » Tue Feb 21, 2012 11:54 am

I just went through this with Valvestorm and Strat78 so I thought I'd summarize the instructions in its own thread.

Equipment required:

Variac (verify the output of the variac is close to what the dial reads)
Preferrably 2 Multimeters - one with plunger clips so that they are shielded and have little chance of popping off the leads on the board. the second with standard pin probes.
100k, 2w resistor

USE CAUTION - THE AMP WILL BE AT FULL TILT AND YOU WILL BE PROBING AROUND LETHAL VOLTAGES/CURRENTS


1) After installing the NOS cap/caps that need to be formed, modify the filter board according to the diagram below. Be sure to insulate the center tap wire to avoid electricution:
Cap forming diagram.jpg
(358.97 KiB) Downloaded 1387 times

2) Attach plunger tip leads on each side of the 100k resistor as shown below and connect to the first multimeter. set the mutimeter to VDC:
forming caps w leads small.jpg
forming caps w leads small.jpg (119.68 KiB) Viewed 2950 times

3) Remove all tubes
4) Turn all control pots to 0.
5) Set the Variac at ~90 VAC and power up the amp.
6) Record the voltage reading across the 100k resistor. Using the second multimeter, carefully measure the voltage across the preamp filter, the PI filter, and coupling caps and take note how close the voltage is to the rated voltage. As the voltage drops across the 100k resistor, that voltage will add to the other filter cap voltages.
7) over a period of a few minutes, gradually ramp the variac voltage up to 120 VAC. Check the voltages across the filters listed above to ensure they are not exceeding the rated voltages.
8 ) The procedure should take between 12-24 hours and is complete when the voltage measured across the 100k resistor is below 5 VDC.
9) As the process comes to completion, its likely that the Variac be turned down to avoid exceding rated cap voltages. Any time you cannot be present to monitor voltages, drop the variac to a level that the math says will be safe.
10) After completion, discharge the board and return it to its previous configuration and reinsert the tubes. Check bias and you're all done.

Cheers

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Re: Cap Forming Procedure for 12xxx Build

Post by vanhalen5150 » Thu Feb 23, 2012 8:28 am

Good info. Where do you get those "plunger clips"? I've never seen those before.
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Re: Cap Forming Procedure for 12xxx Build

Post by YMI5150? » Thu Feb 23, 2012 9:50 am

Radio Shack or any electronics store. I'll see if I can find a link. Definitely safer than alligator clips.

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Re: Cap Forming Procedure for 12xxx Build

Post by jnew » Wed Oct 15, 2014 2:59 pm

This is awesome. I'll be ready for this before much longer. :thumbsup:

Question, since I've bought all vintage RS, Erie, ROE, etc. what might I look for in the event something is dead, damaged, unsafe, etc.?
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Re: Cap Forming Procedure for 12xxx Build

Post by Strat78 » Wed Oct 15, 2014 6:01 pm

We need some stickies like this thread down here in the basement. :thumbsup:

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Re: Cap Forming Procedure for 12xxx Build

Post by ReidarNikolai » Wed Sep 02, 2015 1:27 pm

Can I use the same method without a Variac?

Thanks
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Re: Cap Forming Procedure for 12xxx Build

Post by YMI5150? » Wed Sep 02, 2015 1:35 pm

no, you need to use caution not to exceed the cap ratings so you have to take measurements as you increase VAC.

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Re: Cap Forming Procedure for 12xxx Build

Post by ReidarNikolai » Wed Sep 02, 2015 3:02 pm

OK, so I can´t form caps in a 12 000 without a Variac?

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Re: Cap Forming Procedure for 12xxx Build

Post by jnew » Wed Sep 02, 2015 3:22 pm

It's not recommended to try and form the caps at full voltage from the get go. If the voltage exceeds the caps rating while trying to form, you may have some unwanted problems. Take it as a "NO" for safety. 8)
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Re: Cap Forming Procedure for 12xxx Build

Post by neikeel » Wed Sep 02, 2015 4:34 pm

novosibir wrote:Usually the filter cap manufacturer allow to exceed the rated voltage for 5%, but there's still a safety margin, they don't report about. So to attach a voltage of 475V to a 450V rated filter cap during forming shouldn't be a problem.

If you're afraid about a too high voltage, then put in an additional 470K/1W or a 220K/2W resistor from right after the HT diodes to ground BUT STILL BEFORE THE 100K, this will drop the HT voltage about 10...20V

Larry
So no you do not need a variac if you do this :wink:
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Re: Cap Forming Procedure for 12xxx Build

Post by jnew » Wed Sep 02, 2015 4:52 pm

Or you could do that. :lol:
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Re: Cap Forming Procedure for 12xxx Build

Post by ReidarNikolai » Tue Sep 15, 2015 3:24 am

Hi again.

So I am about to form the caps.
Does this look right? A 470k (to ground) between the diodes and the 100k, the 100k to the start of the caps.
Alligator clips on each side of the 100k - measure?
Image

Thanks for looking
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Re: Cap Forming Procedure for 12xxx Build

Post by ReidarNikolai » Tue Sep 15, 2015 8:22 am

Is the voltage supposed to drift up and down?
A couple of hours is passed - I am monitoring now and then. It is down to 7Vdc +/- - and still going down.

Am I OK?

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Re: Cap Forming Procedure for 12xxx Build

Post by jnew » Tue Sep 15, 2015 9:02 am

I would say things are moving in the right direction. Getting down to 2-3V's would be real nice.
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